The goal of this study is to determine the safety and efficacy of the purified amino acid L-threonine in treating spasticity in patients with multiple sclerosis (MS). MS causes neurologic disability in young adults, and in many individuals spasticity is one of the most disabling symptoms of the disease. The rationale for this study rests upon the anatomical and physiological observations that spinal cord interneurons containing glycine inhibit spinal reflexes. Biochemical data indicate that L- threonine administration increases glycine levels in the spinal cords of rats. Thus, if L-threonine produces similar increases in glycine in the spinal cords of humans, it would be expected to reduce spasticity. Preliminary clinical experience is consistent with this hypothesis. The current proposal is for a double-blind crossover trial of oral L-threonine administration in thirty MS patients with significant spasticity and relatively preserved motor function. Response to treatment will be judged by physician and patient assessments, by standard neurologic measurements of spasticity and function, by electrophysiologic quantitation of spasticity, and by measurement of changes in glycine and other amino acids in cerebrospinal fluid and blood. If L-threonine decreases spasticity and improves function in this study, it will represent an important an important and relatively non-toxic approach to the symptomatic management of MS.